In many applications it is desirable to obtain an analysis of a sample of interest. The analysis desired may be qualitative, quantitative, or both, and may be of only one or of multiple elements or compounds in a sample. For example, in mining applications, such as oil and gas exploration, it is desirable to analyze multiple samples for the presence of one or more minerals. This type of analysis can directly provide analytical information on a mineral which is sought or can provide analytical information on an indicator mineral which may suggest nearby locations of a mineral or other deposit which is sought. For example, by understanding the mineral makeup of a sample, one can identify the possibility that an area being explored is more or less likely to contain oil, gas or oil/gas bearing formations.
X-ray fluorescence (“XRF”) is a technique which has been used for elemental analysis of various samples, including minerals. An XRF analyzer determines the chemistry of a sample by illuminating a spot on the sample with x-rays and measuring the spectrum of characteristic x-rays emitted by the different elements in the sample. The primary source of x-rays may be an x-ray tube or a radioactive material, such as a radioisotope. The term “x-rays” as used herein, includes photons of energy between about 1 keV and about 150 keV and will, therefore, include: the characteristic x-rays emitted by an excited atom when it deexcites; bremsstrahlung x-rays emitted when an electron is scattered by an atom; elastic and inelastically scattered photons generally referred to as Rayleigh and Compton scattered radiation, respectively.
When exposed to high energy primary x-rays from a source, each atomic element present in a sample produces a unique set of characteristic fluorescence x-rays that are essentially a fingerprint for the specific element. An x-ray fluorescence analyzer determines the chemistry of a sample by illuminating a spot on the sample with x-rays and measuring the spectrum of characteristic x-rays emitted by the various elements in the sample. The primary source of x-rays may be an x-ray tube or a radioactive material, such as a radioisotope. At the atomic level, a characteristic fluorescent x-ray is created when a photon of sufficient energy strikes an atom in the sample, dislodging an electron from one of the atom's inner orbital shells. The atom then nearly instantaneously regains stability, filling the vacancy left in the inner orbital shell with an electron from one of the atom's higher energy (outer) orbital shells. Excess energy may be released in the form of a fluorescent x-ray, of an energy characterizing the difference between two quantum states of the atom. By inducing and measuring a wide range of different characteristic fluorescent x-rays emitted by the different elements in the sample, XRF analyzers are able to determine the elements present in the sample, as well as to calculate their relative concentrations based on the number of fluorescent x-rays occurring at specific energies. However, except in special circumstances, low concentrations of light elements (those with low atomic number, Z, typically below 20) cannot typically be measured directly with portable XRF analyzers because fluorescent x-rays with energies below about 2.5 kiloelectron volts (keV) are absorbed within short path lengths of air. For this reason, light element XRF analysis requires either a helium gas purge or the evacuation of the volumes through which the relevant x-rays pass, which can be inconvenient for a portable or hand-held instrument.
XRF analyzers are well known, and include those described in U.S. Pat. Nos. 7,875,847, 7,916,834, and 7,791,027, which are incorporated herein by reference.